Plants are explicitly described as non-sentient by most philosophers. Some of them are even quite crude in denying any sentience to trees. Why? Probably because they had to contrast popular "superstitions" according too much importance to plants, especially to trees.
In many of the early Buddhist legends, drawn from such sources as the Jātakas, the Dhammapadatthakathā, the Mahāvastu, the Dīpavaṃsa, the Divyāvadāna, the Aśokāvadāna, and so forth, an important dialectic is set up between the morally and spiritually perfected Buddha and various nonhuman deities such as yakṣas (Pāli, yakkhas) and the serpent deities, the nāgas. On the one hand, the Buddha incorporates and presides over a pre-existing mythology of nature. In so doing, the new religion of Buddhism is able more readily to meet the needs of an unlettered laity (Sutherland, Yakṣa in Hinduism and Buddhism p. 26).(I would say that the same process applies to many "Hindu" religions. For instance, Arjuna marries, among other women, a nāga-princess, and his brother a Rākṣasī.) Then, philosophers had to sort out a consistent theology out of these inclusive processes and, thus, tended to deny the sentience of plants. Buddhist may have been even more harsh in this process (see Schmithausen's illuminating essays on this topic) due to the typically "Buddhist dialectic between nature and ascetism" (Sutherland, p. 28).